Support ARM_V4BX relocation

Summary: This patch implementation the handler for ARM_V4BX. This relocation is used by GNU runtime files and other armv4 applications.

Patch by Yin Ma

Reviewers: espindola, MaskRay, ruiu, peter.smith, pcc

Reviewed By: ruiu

Subscribers: yinma, pcc, peter.smith, MaskRay, rovka, efriedma, emaste, javed.absar, arichardson, kristof.beyls, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D53444

llvm-svn: 347077